1. Field of the Invention
The present invention relates to an apparatus and method for sensing a rotor position and an electric power steering system using the same. More particularly, the present invention relates to an apparatus and method for sensing a rotor position, and an electric power steering system using the same wherein resolution in rotor position recognition can be improved by software by adding an increment value, which is proportional to the rotation speed of a rotor, in an interval where the counted number of increment pulses of a pulse generator is not changed in sampling the counted number of increment pulses of the pulse generator so as to recognize a rotor position of a motor.
2. Description of the Prior Art
As generally known in the art, an EPS (Electric Power Steering) system drives a motor, through an ECU (Electronic Control Unit), depending on a driving condition of a vehicle determined by a vehicle speed sensor, a steering angle sensor, a steering torque sensor, etc., in such a manner that the EPS system imparts, at the time of low-speed driving, light and convenient steering feeling, and at the time of high-speed driving, heavy steering feeling and good directional stability to a driver, whereby the driver can be provided with the optimal steering condition.
The above-mentioned electric power steering system is configured in such a manner that a motor is installed in the outside of a steering column to be capable of rotating a steering shaft positioned within the steering column, wherein the steering column is interposed between steering wheel and a gear box so as to transmit force applied by the driver for rotating the steering wheel to a lower side. As a result, the electric power steering system is adapted to support the driver's steering force according to the steering of the steering wheel.
FIG. 1 is a schematic view of an electric power steering system.
As shown in FIG. 1, the electric power steering system 100 includes a steering network 130 extending from a steering wheel 102 to left and right front wheels 126, and an auxiliary power mechanism 140 for supplying steering support power to the steering network 130.
The steering network 130 includes a steering shaft 106, one end of which is connected to the steering wheel 102 to be rotated with the steering wheel 102, and the other end of which is connected to a pinion shaft 108 through a pair of universal joints 104. In addition, the pinion shaft 108 is connected to a rack bar 112 through a rack and pinion mechanism 110, and the opposite ends of the rack bar 112 are connected to the wheels 126 of the vehicle through tie rods 122 and knuckle arms 124.
The auxiliary power mechanism 140 includes a torque sensor 142 for sensing torque applied to the steering wheel 102 by the driver and outputting an electric signal proportional to the sensed torque, an angle sensor 143 for outputting an electric signal which is proportional to the rotated angle of the steering wheel, an electronic control unit 144 for generating a control signal on the basis of the electric signals transmitted from the torque sensor 142 and the angle sensor 143, a motor 146 for generating auxiliary power on the basis of the control signal transmitted from the electronic control unit 144, and a speed reducer 150 having a worm gear 152 and a worm wheel gear 156 so as to transmit the auxiliary power generated by the motor to the steering shaft 106.
A function for precisely sensing the rotor position of the motor is required when the ECU of the EPS drives the motor, and the rotor position is sensed using an absolute position sensor for the rotor and a pulse generator.
FIG. 2 exemplifies output of an absolute position sensor and output of a pulse generator.
Referring to FIG. 2, the absolute position sensor of a 3-phase motor is divided into six sections by an absolute position sensor for each of U, V and W phases, and the pulse generator generates N increment pulses per one period of the absolute position sensor, wherein “N” means the number of increment pulses.
In the past, the rotor position was calculated using only absolute position information and the number of increment pulses counted in synchronization with the absolute position information. Each time when the number of increment pulses is increased by one (1), the rotor position is renewed. In accordance with this method, if a sampling period is set in such a manner that sampling for sensing the position of the rotor is executed several times within one period of the increment pulses, no change may be caused in sampling value even if the rotor is rotated.
For example, assuming that the number of increment pulses is counted at rising edges of the increment pulses in FIG. 2, there is a problem in that if sampling is executed at time points a, b and c, the sampling value is changed between the time point a and the time point b because the counted number of increment pulses is changed but between the time point b and the time point c, the rotor position is sensed as being unchanged because the number of counted increment pulses is not changed.
In addition, if the motor is controlled by the above-mentioned method, a problem arises in that harmonic noise corresponding to N times of the number of mechanical rotations of the rotor is produced, thereby producing noise sounds.